Life and death under the sea - an Indian Ocean detective story

At 2 am on Saturday, 29 November 2014, two WWF staff headed out to sea from the tropical Indian Ocean island of Mahe. On board separate sport fishing boats, the two were part of the inaugural Seychelles Great Marlin Race, a ‘citizen-science’ initiative tapping into the skills of the recreational sports anglers to capture blue marlin and attach sophisticated electronic tags to these spectacular fish. This tournament is well known in the Pacific and Atlantic but has never taken place in the Indian Ocean, until now.

The Great Marlin Race, supported by WWF in partnership with Seychelles Sports Fishing Club to promote science and conservation, utilises these electronic tags to follow the movements of highly migratory blue marlin. Each tag is programmed to automatically release approximately 6 months following deployment. When the tags float to the surface and are located by an orbiting satellite, the marlin that is furthest from Mahe is declared the winner of the Great Marlin Race.

But while the race is over for the fishermen, for the scientists the real work then starts. After reaching the surface each tag sends data back to researchers about where the fish went and what it did during the deployment. This gives unprecedented insights into the way these animals live in the open ocean.

The first electronic tag on a marlin surfaced after only 5 days …

WWF donated two of these tags to the tournament. One was successfully deployed on a 45kg blue marlin and hopes were high for a long and productive time at sea gathering data on the movements of the fish. However, after only 5 days the tag surfaced, reported its position and sent a stream of intriguing data back to the scientists.

For the first 5 hours, data suggested all was well. The water temperature detected by the tag changed as the fish moved up and down in the sea, getting cooler as the fish swam deeper, and warmer nearer the surface. Then suddenly both the temperature and depth patterns changed and, more tellingly, the tag stopped detecting any light. Researchers came to the conclusion that the very expensive tag was now in the stomach of an unknown, and possibly quite large, predator.

This predator then apparently continued its normal activities, staying at relatively shallow depths for the next few days with one notable 30 minute dive just after midnight on 2 December to a depth of over 1 km, providing another clue in the unfolding detective story. As the temperature dropped in relation to the dive, scientists believe this indicates the odd data point is valid and not just a glitch in the instrument. Eventually, on 4 December, the tag passed through its mysterious host and floated to the surface to report on its journey.

…and sent intriguing data to the scientists

Scientists at Stanford University, also partners in the Great Marlin Race, do not believe the culprit was a mako or other lamnid species of shark as these are able to maintain high core body temperatures compared to other ‘cold blooded’ fish. They suggested that perhaps the predator was a large oceanic whitetip shark.

WWF’s own shark expert, Andy Cornish, immediately ruled out any cetacean suspects, such as toothed whales due to the temperature pattern, but wasn’t as quick in eliminating the mako shark. He notes they can stay warmer than the cold water around them, but do not necessarily maintain constant temperatures and the recorded fluctuations are within accepted temperature parameters for these animals. However he did make the observation that deep dives are not typical behavior for makos but that oceanic whitetip sharks are known to make deep dives greater than 1000m, often at night.

But there is also an alternate theory. A large yellowfin tuna may have been attracted by the alluring movement of the tag and snatched it from the back of the marlin. These tuna can also exhibit the deep dives, thought to possibly be attempts to dislodge parasites.

While the truth may never be known for certain, the first tag returned from this study painted a fascinating picture of life and death under the sea. There are more tags still at large in the Indian Ocean and more to be deployed during future tournaments which will be held twice every year, providing a wealth of information to scientists attempting to unravel the lives and migratory patterns of these key predatory species.